Selectable capacitance circuit

ABSTRACT

A voltage-controlled capacitor and methods for forming the same are described. A mechanical conductor membrane of the voltage-controlled capacitor is movable to and from a first position and a second position. An amount of capacitance can vary with the movement of the mechanical conductor membrane. A microelectromechanical systems (MEMS) voltage-controlled capacitor can be used in a variety of applications, such as, but not limited to, RF switches and RF attenuators.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Application No. 60/613,409, filed Sep. 27, 2004, theentirety of which is hereby incorporated by reference.

This application is a continuation-in-part application of U.S.application Ser. No. 11/134,222, filed May 20, 2005, the entirety ofwhich is incorporated by reference herein, which also claims the benefitunder 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/613,409,filed Sep. 27, 2004.

BACKGROUND

1. Field of the Invention

The invention generally relates to microelectromechanical systems(MEMS).

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. These MEMSdevices can be used in a variety of applications, such as in opticalapplications and in electrical circuit applications.

One type of MEMS device is called an interferometric modulator. As usedherein, the term interferometric modulator or interferometric lightmodulator refers to a device that selectively absorbs and/or reflectslight using the principles of optical interference. In certainembodiments, an interferometric modulator may comprise a pair ofconductive plates, one or both of which may be transparent and/orreflective in whole or part and capable of relative motion uponapplication of an appropriate electrical signal. One plate may comprisea stationary layer deposited on a substrate, the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

Another type of MEMS device is used as a multiple-state capacitor. Forexample, the capacitor can comprise a pair of conductive plates with atleast one plate capable of relative motion upon application of anappropriate electrical control signal. The relative motion changes thecapacitance of the capacitor, permitting the capacitor to be used in avariety of applications, such as a filtering circuit, tuning circuit,phase-shifting circuit, an attenuator circuit, and the like.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly.

One embodiment is an apparatus having a selectable amount ofcapacitance, the apparatus including: at least two electrodes at leastone of which is controllably movable with respect to the other toprovide adjustability of a gap defined between the at least twoelectrodes, wherein at least one of the electrodes carries an RF signal;and a plurality of posts disposed between the at least two electrodes,wherein the plurality of posts are configured to tension at least one ofthe electrodes.

One embodiment is a capacitor having a selectable amount of capacitance,the capacitor including: means for carrying an RF signal, the carryingmeans having a controllable adjustable gap; and means for tensioning atleast part of the carrying means.

One embodiment is a method of selecting capacitance, the methodincluding: adjusting a gap between at least two electrodes, wherein atleast one of the electrodes carries an RF signal; and tensioning atleast one of the electrodes.

One embodiment is a method of manufacturing a capacitor having aselectable capacitance, the method including: forming a first electrode;forming a second electrode such that it is movable with respect to thefirst electrode to provide adjustability of a gap defined between thefirst electrode and the second electrode; and forming a plurality ofposts configured to tension at least the second electrode, wherein theposts are disposed between the electrodes.

One embodiment is a capacitor produced in accordance with the foregoing.

One embodiment is an RF device, the RF device including: a firstconductor for carrying an RF signal; and a deformable membrane spacedapart from the first conductor, the deformable membrane configured toselectively filter the RF signal, the deformable membrane having atleast three discrete actuatable positions for selectively filtering theRF signal.

One embodiment is an RF device, including: means for carrying an RFsignal; and means for filtering the RF signal, the filtering means beingdeformable to at least one of three discrete actuatable positions toselectively filter the RF signal.

One embodiment is a method of filtering an RF signal, the methodincluding: carrying the RF signal in a conductive line; and selectivelyfiltering the RF signal using a deformable membrane having at leastthree discrete actuatable positions for selectively filtering the RFsignal, wherein the deformable membrane is adjacent to the conductiveline.

One embodiment is a method of manufacturing an RF device having aselectable capacitance, the method including: forming a first conductorfor carrying an RF signal; and forming a deformable membrane spacedapart from the first conductor, the deformable membrane configured toselectively filter the RF signal, the deformable membrane having atleast three discrete actuatable positions for selectively filtering theRF signal.

One embodiment is an RF device produced in accordance with the methoddescribed in the foregoing.

One embodiment is a voltage-controlled capacitor, the voltage-controlledcapacitor-including: a substrate assembly with an input terminal, acontrol terminal, and a voltage reference terminal; voltage referencelines disposed on the substrate assembly, wherein at least one of thevoltage reference lines is coupled to the voltage reference terminal; amechanical conductor membrane spaced above the substrate assembly andcoupled to one or more of the voltage reference lines at opposing endsof the mechanical conductor membrane; one or more posts disposed betweenthe substrate assembly and the mechanical conductor membrane, whereinthe one or more posts support the mechanical conductor membrane; asignal conductor disposed on the substrate assembly, wherein a voltageon the control terminal at least partially controls the position of themechanical conductor membrane; a layer of dielectric material disposedbetween a top surface of the signal conductor and the mechanicalconductor membrane; and a coupling capacitor with a first terminal and asecond terminal, wherein the first terminal is coupled to the inputterminal and wherein the second terminal is coupled to the signalconductor.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings (not to scale) and the associated description herein areprovided to illustrate embodiments and are not intended to be limiting.

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D illustrates a cross-sectional side view of a MEMS capacitor witha mechanical conductor membrane in a low-capacitance position.

FIG. 7E illustrates a cross-sectional side view of the MEMS capacitor ofFIG. 7D with the mechanical conductor membrane in a high-capacitanceposition.

FIG. 8 illustrates a cross-sectional side view of a MEMS capacitoraccording to one embodiment where the membrane is insulated from avoltage reference.

FIG. 9 illustrates a top view of an embodiment of a MEMS capacitor witha relatively uniform post spacing for the membrane.

FIG. 9B1 illustrates a top view of an embodiment of a MEMS capacitorwith relatively wide post spacing for a first portion of the membraneand a relatively tight post spacing for a second portion of themembrane.

FIG. 9B2 illustrates a top view of another embodiment of a MEMScapacitor with relatively wide post spacing for a first portion of themembrane and a relatively tight post spacing for a second portion of themembrane.

FIG. 9C1 illustrates a top view of an embodiment of a MEMS capacitorwith two separate membranes and with different post spacing for eachmembrane.

FIG. 9C2 illustrates a top view of another embodiment of a MEMScapacitor with two separate membranes and with different post spacingfor each membrane.

FIG. 9D illustrates a top view of an embodiment of a MEMS capacitor withtwo separate membranes and the same post spacing for each of theillustrated membranes.

FIG. 10A illustrates an example of an expected return loss for an RFattenuator using a MEMS capacitor.

FIG. 10B illustrates an example of an expected insertion loss for an RFattenuator using a MEMS capacitor.

FIG. 11 illustrates an example of a MEMS capacitor in an RF attenuator.

FIGS. 12A, 12B, and 12C illustrate examples of simplified equivalentcircuits for a MEMS capacitor.

FIGS. 13A to 13I illustrate a process to fabricate a MEMS capacitor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Although particular embodiments are described herein, other embodiments,including embodiments that do not provide all of the benefits andfeatures set forth herein, will be apparent to those of ordinary skillin the art.

A selectable capacitance circuit can be used in a wide variety ofapplications. For example, the selectable capacitance circuit can beused in an RF attenuator or in an RF switch. The selectable capacitancecan be used to select an amount of RF attenuation, to select an amountof impedance mismatch for an RF switch, and the like. An attenuator or aswitch fabricated from a MEMS device advantageously exhibits relativelywide-bandwidth operation with relatively low-loss and superior RFcharacteristics in comparison to diode and FET switches. MEMS devicesalso typically require relatively low drive power and can exhibitrelatively low series resistance.

While generally described with reference to FIGS. 1 to 6C in the contextof an interferometric modulator display, the skilled artisan willappreciate that the principles of the relative movement of one or bothof the conductive plates or membranes of a MEMS device for a displaywill also be applicable to a MEMS capacitor.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theembodiments may be implemented in any device that is configured todisplay an image, whether in motion (e.g., video) or stationary (e.g.,still image), and whether textual or pictorial. More particularly, it iscontemplated that the embodiments may be implemented in or associatedwith a variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a relaxed position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers 14 a, 14 b are separated from the fixed metal layers by adefined gap 19. A highly conductive and reflective material such asaluminum may be used for the deformable layers, and these strips mayform column electrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate aspects of the invention. In theexemplary embodiment, the electronic device includes a processor 21which may be any general purpose single- or multi-chip microprocessorsuch as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®,Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any specialpurpose microprocessor such as a digital signal processor,microcontroller, or a programmable gate array. As is conventional in theart, the processor 21 may be configured to execute one or more softwaremodules. In addition to executing an operating system, the processor maybe configured to execute one or more software applications, including aweb browser, a telephone application, an email program, or any othersoftware application.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a display array or panel 30. The cross section of thearray illustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. ForMEMS interferometric modulators, the row/column actuation protocol maytake advantage of a hysteresis property of these devices illustrated inFIG. 3. It may require, for example, a 10 volt potential difference tocause a movable layer to deform from the relaxed state to the actuatedstate. However, when the voltage is reduced from that value, the movablelayer maintains its state as the voltage drops back below 10 volts. Inthe exemplary embodiment of FIG. 3, the movable layer does not relaxcompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the relaxed or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be relaxed areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or relaxed pre-existingstate. Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Relaxing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, it will be appreciated that voltages of opposite polarity than thosedescribed above can be used, e.g., actuating a pixel can involve settingthe appropriate column to +V_(bias), and the appropriate row to −ΔV. Inthis embodiment, releasing the pixel is accomplished by setting theappropriate column to −V_(bias), and the appropriate row to the same−ΔV, producing a zero volt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or relaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thesystems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 44, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processesas are well known to those of skill in the art, including injectionmolding, and vacuum forming. In addition, the housing 41 may be madefrom any of a variety of materials, including but not limited toplastic, metal, glass, rubber, and ceramic, or a combination thereof. Inone embodiment the housing 41 includes removable portions (not shown)that may be interchanged with other removable portions of differentcolor, or containing different logos, pictures, or symbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device, as is wellknown to those of skill in the art. However, for purposes of describingthe present embodiment, the display 30 includes an interferometricmodulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. [The conditioning hardware 52 may beconfigured to condition a signal (e.g. filter a signal).] Theconditioning hardware 52 is connected to a speaker 44 and a microphone46. The processor 21 is also connected to an input device 48 and adriver controller 29. The driver controller 29 is coupled to a framebuffer 28, and to an array driver 22, which in turn is coupled to adisplay array 30. A power supply 50 provides power to all components asrequired by the particular exemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one oremore devices over a network. In one embodiment the network interface 27may also have some processing capabilities to relieve requirements ofthe processor 21. The antenna 43 is any antenna known to those of skillin the art for transmitting and receiving signals. In one embodiment,the antenna transmits and receives RF signals according to the IEEE802.11 standard, including IEEE 802.11(a), (b), or (g). In anotherembodiment, the antenna transmits and receives RF signals according tothe BLUETOOTH standard. In the case of a cellular telephone, the antennais designed to receive CDMA, GSM, AMPS or other known signals that areused to communicate within a wireless cell phone network. Thetransceiver 47 pre-processes the signals received from the antenna 43 sothat they may be received by and further manipulated by the processor21. The transceiver 47 also processes signals received from theprocessor 21 so that they may be transmitted from the exemplary displaydevice 40 via the antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 44, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. Those of skill in the art will recognizethat the above-described optimization may be implemented in any numberof hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7C illustrate three different embodiments of themoving mirror structure. FIG. 7A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 7B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 7C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of known techniques may be used to producethe above described structures involving a series of materialdeposition, patterning, and etching steps.

A microelectromechanical systems (MEMS) voltage-controlled capacitor andmethods for forming the same are described. A mechanical conductormembrane of the voltage-controlled capacitor is movable to and from afirst position and a second position. An amount of capacitance can varywith the movement of the mechanical conductor membrane. A MEMSvoltage-controlled capacitor can be used in a variety of applications,such as, but not limited to, RF switches and RF attenuators.

An attenuator or a switch fabricated from a MEMS device advantageouslyexhibits relatively wide-bandwidth operation with relatively low-lossand superior RF characteristics in comparison to diode and FET switches.Further, these MEMS devices can also feature relatively low drive powerand relatively low series resistance where used in coplanar waveguides.

One embodiment includes a MEMS capacitor with posts disposed betweenanchoring points of the membrane. The spacing of the posts can determinea pull-in voltage used to change the position of the membrane. Acapacitor can be formed with one or more membranes having varying postspacing. This permits the pull-in voltage to vary for correspondingportions of membranes, thereby permitting the selective actuation ofmembranes or portions thereof. Accordingly, the amount of capacitancecan vary at least partially in response to the control voltage.

One embodiment includes a capacitor with multiple membranes that arecoupled to separate control biases. This permits the independent controlof the multiple membranes, thereby allowing a relatively large range ofcapacitance to be selected. For example, the multiple membranes can beweighted in binary weights (powers of 2) to provide near linearselection of capacitance.

One embodiment is a voltage-controlled capacitor including: a substrateassembly with an input terminal, a control terminal, and a voltagereference terminal; voltage reference lines disposed on the substrateassembly, wherein at least one of the voltage reference lines is coupledto the voltage reference terminal; a mechanical conductor membranespaced above the substrate assembly and coupled directly or indirectlyto one or more of the voltage reference lines at opposing ends of themechanical conductor membrane so that the opposing mechanical conductormembrane is anchored at two or more ends and so that the mechanicalconductor membrane is AC coupled to the one or more voltage referencelines, wherein at least a portion of the mechanical conductor membraneis movable to and from a first position a first distance from a surfaceof the substrate assembly and a second position a second distance fromthe surface of the substrate assembly; one or more posts disposedbetween the substrate assembly and the mechanical conductor membrane anddisposed between the two or more ends anchoring the mechanical conductormembrane, where the one or more posts support the mechanical conductormembrane; a signal conductor disposed on the substrate assembly, wherethe signal conductor is DC coupled to the control terminal; wherein avoltage on the control terminal at least partially controls the positionof the mechanical conductor membrane; a layer of dielectric materialdisposed between a top surface of the signal conductor and themechanical conductor membrane, where a gap exists between at least oneof (a) the mechanical conductor membrane and the layer of dielectricmaterial or (b) the layer of dielectric material and the signalconductor when the mechanical conductor membrane is in the firstposition, and substantially no gap exists when the mechanical conductormembrane is in the second position; and a coupling capacitor with afirst terminal and a second terminal, where the first terminal iscoupled to the input terminal and where the second terminal is coupledto the signal conductor.

One embodiment is a capacitor having a selectable capacitance, thecapacitor including: a substrate assembly; a signal conductor on thesubstrate assembly, wherein the signal conductor forms a first electrodefor the capacitor; a layer of dielectric material covering at least anupper surface of the signal conductor; and one or more mechanicalconductor membranes spaced above the substrate assembly such that thesignal conductor is disposed between the substrate assembly and the oneor more mechanical conductor membranes, where the one or more mechanicalconductor membranes form a second electrode for the capacitor, whereinat least two or more portions of the one or more mechanical conductormembranes are at least partially independently movable from a lowcapacitance position and a high capacitance position, such attainablepositions include a discrete first capacitance position for at least aselected two portions of the mechanical conductor membranes, a discretesecond capacitance position for the selected two portions of themechanical conductor membranes, the discrete second capacitance positionhaving more capacitance than the discrete first capacitance position,and a discrete third capacitance configuration having more capacitancethan the discrete first capacitance position but less capacitance thanthe discrete second capacitance position, in the discrete thirdcapacitance configuration, one of the selected two portions is in thediscrete second capacitance position and the other is in the discretefirst capacitance position, wherein the selected position is at leastpartially determined by a voltage on the signal conductor.

FIG. 7D illustrates a cross-sectional side view of a MEMS capacitor 700with a mechanical conductor membrane 702 in a low capacitance position.FIG. 7E illustrates the same MEMS capacitor 700 in a high capacitanceposition. One process for fabricating the MEMS capacitor 700 will bedescribed later in connection with FIGS. 13A to 13I. The MEMS capacitor700 also includes a substrate assembly 704, voltage reference lines 706,708, posts 710, a signal conductor 712, and a layer of dielectricmaterial 714 disposed on the signal conductor 712.

In the illustrated embodiment, the voltage reference lines 706, 708 andthe signal conductor 712 are formed on the substrate assembly 704 in acoplanar waveguide configuration. It will be understood that otherstructures, such as barrier layers, can also be present. Of course, thematerial for a barrier layer will depend on the materials used for thevoltage reference lines 706, 708. For example, where the voltagereference lines 706, 708 are formed from copper, tantalum can be used asa diffusion barrier. The substrate assembly 704 can be formed from avariety of materials, such as glass, silicon, gallium arsenide, lithiumniobate, indium phosphide, and the like. It should be noted that unlikethe materials that should be used in an interferometric modulator for adisplay application, the materials used for the substrate assembly 704,the voltage reference lines 706, 708, and the signal conductor 712 donot need to be selected for relatively good transparency in the humanvisible spectrum. Rather, the materials can be selected based onelectrical performance characteristics, cost, and the like. Examples ofmaterials that can be used for the voltage reference lines 706, 708 andfor the signal conductor 712 include silver, copper, gold, aluminum, orcombinations thereof. In one embodiment, the material used for thevoltage reference lines 706, 708 and for the signal conductor 712 is thesame. The selected material is preferably a relatively good conductor,such as a material having a resistivity of less than 1×10⁻⁶ ohm-meters(Ω-m) or even more preferably, less than 0.1×10⁻⁶ ohm-meters (Ω-m).

The voltage reference lines 706, 708 provide a signal ground referencefor the signal carried by the signal conductor 712. The signal groundshould provide a relatively low impedance to ground for RF signals. Itwill be understood that such a signal ground can be, but does not haveto be, at DC ground potential. In the embodiment illustrated in FIGS. 7Dand 7E, the voltage reference lines 706, 708, and the mechanicalconductor membrane 702 are at the same DC potential. In an embodimentthat will be described later in connection with FIG. 8, different DCpotentials can be used.

The signal conductor 712 carries the signal for which a selectablecapacitance is provided. For example, the selectable capacitance can beused in an RF attenuator to select an amount of attenuation applied tothe signal, can be used in an RF switch to select a path for the signal,and the like. A coupling capacitor can be used to isolate the RF signalfrom a control voltage that is also carried by the signal conductor 712.The control voltage can at least partially control the position of themechanical conductor membrane 702 as described earlier in connectionwith FIG. 3.

In the illustrated embodiment, the layer of dielectric material 714 isformed on the signal conductor 712. In another embodiment, the layer ofdielectric material 714 can be disposed on the bottom side (side facingthe signal conductor 712) of the mechanical conductor membrane 702. Avariety of materials can be used for the layer of dielectric material714, such as, for example, silicon oxide, silicon nitride, and the like.The layer of dielectric material 714 prevents the mechanical conductormembrane 702 and the signal conductor 712 from electrically shortingwhen in the low capacitance position illustrated in FIG. 7E.

The mechanical conductor membrane 702 should also be formed from aconductive material. A wide variety of materials can be used. Forexample, the same materials used for the voltage reference lines 706,708 and for the signal conductor 712 can be used. In addition, themechanical conductor membrane 702 can also be formed from multiplelayers of various materials selected to provide relatively goodelectrical and mechanical properties, such as stress.

Posts 710 can be formed from a variety of materials (conductive ordielectric), such as from polymers, metals, glasses, ceramics, and thelike. In one embodiment, the posts 710 are formed from a photo-sensitivepolymer for ease of fabrication. The posts 710 support the mechanicalconductor membrane 702 such that in the low capacitance position, themechanical conductor membrane 702 is a height h above a surface of thesubstrate. The height of the posts 710 (also h), the spacing betweenposts 710, and the tensile stress on the mechanical conductor membrane702 can be used to select an appropriate pull-in voltage for themechanical conductor membrane 702.

It will be understood by the skilled practitioner that the appropriatematerials and dimensions to use for a particular MEMS capacitor 700 willdepend on a variety of considerations such as cost, electricalperformance requirements (such as frequency range), available size,desired pull-in voltages, and the like. In one embodiment, anappropriate thickness for the conductors for the voltage reference lines706, 708 and for the signal conductor 712 is in a range of about 0.5 to5 micrometers. An appropriate width w for the signal conductor 712 is ina range of about 25 micrometers to about 75 micrometers. An appropriatewidth L for the voltage reference lines 706, 708 is in a range of about50 micrometers to about 250 micrometers. An appropriate distance gbetween one of the voltage reference lines 706, 708 and the signalconductor 712 is in a range of about 10 micrometers to about 50micrometers. In one embodiment, an appropriate thickness for the layerof dielectric material 714 is in a range of about 0.1 to 0.5micrometers. Other appropriate dimensions will be readily determined byone of ordinary skill in the art.

The mechanical conductor membrane 702 can move to and from a firstposition and a second position. As illustrated in FIG. 7D, there is agap between the bottom of the mechanical conductor membrane 702 and thelayer of dielectric material 714. The presence of this gap provides theMEMS capacitor 700 with relatively low capacitance in the positionillustrated in FIG. 7D. When activated by an appropriate pull-in voltagebetween the mechanical conductor membrane 702 and the signal conductor712, the mechanical conductor membrane 702 moves to a higher capacitanceposition as illustrated in FIG. 7E.

FIG. 8 illustrates a cross-sectional side view of a MEMS capacitor 800according to one embodiment where a layer of dielectric material 802insulates a mechanical conductor membrane 804 from a voltage reference.The layer of dielectric material 802 is disposed between the mechanicalconductor membrane 804 and voltage reference lines 706, 708. Thispermits the voltage reference lines 706, 708 to be at a different DCelectric potential than the mechanical conductor membrane 804. Themechanical conductor membrane 802 can be extended to contact a sourcefor the DC bias as shown to the right of FIG. 8. It should be noted thatone of or both voltage reference lines 706, 708 should still be coupledto a relatively good signal ground.

A wide variety of materials can be used for the layer of dielectricmaterial 802. For example, the layer of dielectric material 802 can beformed from aluminum oxide, silicon oxide, silicon nitride, and thelike. In one embodiment, the voltage reference line 708 is coupled to aDC ground, and the mechanical conductor membrane 804 is coupled to a DCbias relative to the bias on the signal conductor 712 for actuation ofthe position of the mechanical conductor membrane 804. This can permit,for example, DC isolated sections of a mechanical conductor membrane tobe selectively activated or moved, thereby providing a relatively widerange of selectable capacitance. This can be useful in an RF attenuationapplication. In one example, the signal conductors and the mechanicalconductor membranes are arranged in rows and columns and activated asdescribed earlier in connection with FIGS. 5A and 5B.

FIG. 9 illustrates a top view of an embodiment of a MEMS capacitor 900having a relatively uniform post spacing. For example, the top view ofthe MEMS capacitor 900 can correspond to the MEMS capacitor 800described earlier in connection with FIG. 8. The illustrated portions ofthe MEMS capacitor 900 include voltage reference lines 902, 904, signalconductor 906, and posts 908. A dashed box 910 indicates a top view ofthe mechanical conductor membrane. In the illustrated embodiment, thedashed box 910 is drawn extending beyond the voltage reference line 904for coupling to a source of a DC potential for biasing of the mechanicalconductor membrane.

In one embodiment, where the capacitor is embodied in an RF attenuatoror in an RF switch in a coplanar waveguide configuration, the RF signalcan flow across the capacitor such that an RF input signal and an RFoutput signal can be coupled to terminals at opposing ends of the signalconductor 906. Such coupling can be, for example, via a couplingcapacitor or other coupling that does not pass DC from a source of acontrol voltage.

With relatively consistent or uniform spacing, the entire movableportion of the mechanical conductive membrane can be expected to movefrom one position to another substantially simultaneously with itself.

FIG. 9B1 illustrates a top view of an embodiment of a MEMS capacitorwith relatively wide post spacing for a first portion 912 of themechanical conductor membrane 916 and relatively tight post spacing fora second portion 914 of the mechanical conductor membrane 916. FIG. 9B2illustrates a top view of another embodiment of a MEMS capacitor withrelatively wide post spacing for the first portion 912 of the mechanicalconductor membrane 916 and relatively tight post spacing for the secondportion 914 of the mechanical conductor membrane 916. A dashed line 918is drawn approximately between the two portions.

It should be noted that although the mechanical conductor membrane 916is in one piece such that the first portion 912 and the second portion914 are portions of the same mechanical conductor membrane 916, thefirst portion 912 and the second portion 914 can independently move. Byvarying the heights (not shown) and/or the spacing between the posts,the pull-in voltage required can vary between the different portions.For example, with the same height for both the first portion 912 and thesecond portion 914, the first portion 912 will pull in at a loweractuation voltage than the second portion 914. In the embodiment of FIG.9B1, the spacing varies in a direction parallel to the signal conductor.In the embodiment illustrated in FIG. 9B2, each column of posts 952 isspaced closer to a respective signal conductor 956 in the second portion914 than are each column of posts 954 in the first portion 912.

Although two portions are shown in FIGS. 9B1 and 9B2, it will beunderstood that more portions, such as 3, 4, or more can be used. In oneembodiment, the posts beneath the multiple portions of a mechanicalconductor membrane 916 are arranged according to the desiredselectability in capacitance.

FIG. 9C1 illustrates a top view of an embodiment of a MEMS capacitorwith two separate membranes 922, 924 and with different post spacing foreach membrane. FIG. 9C2 illustrates a top view of another embodiment ofa MEMS capacitor with two separate membranes 922, 924 and with differentpost spacing for each membrane. For example, while the separatemembranes 922, 924 can be tied to the same DC bias provided by commonvoltage reference lines, the membranes 922, 924 can actuate at differentpull-in voltages thereby providing multiple selectivity of capacitancevalues. It will be understood that additional separate membranes canalso be provided to provide additional selectability of capacitance. Inthe embodiment illustrated in FIG. 9C2, each column of posts 962 isspaced closer to a respective signal conductor 966 for the secondmembrane 924 than are each column of posts 964 of the first membrane922.

FIG. 9D illustrates a top view of an embodiment of a MEMS capacitor withtwo separate mechanical conductor membranes 932, 934 and the same postspacing for the illustrated mechanical conductor membranes. Thisconfiguration can provide additional control over the configurationsdescribed earlier in connection with FIGS. 9B1, 9B2, 9C1, and 9C2.

By using separate control biases on each of the illustrated mechanicalconductor membranes 932, 934, each of the membranes 932, 934 can beindependently pulled-in. These separate control biases are in additionto the control bias on the signal conductor. It will be understood thatone of the separate control biases can correspond to ground. Thisincreases the selectability provided by the capacitor. For example, thedifferent mechanical conductor membranes 932, 934 can bebinary-weighted, that is, approximately in powers of two by area. Thiscan permit the amount of capacitance to be nearly linearly controlled.It should be noted that it may be necessary in some situations to movethe membranes 932, 934 back to a low capacitance position betweenselected capacitance values. While illustrated in the context of twoseparate membranes 932, 934, the skilled practitioner will appreciatethat additional numbers of membranes can be used.

The separate membranes 932, 934 can be isolated from each other'scontrol voltage. For example, the configuration described earlier inconnection with FIG. 8 illustrates such an isolation technique with thelayer of dielectric material 802. With reference to FIG. 9D, adielectric layer 936 can isolate one or more of the membranes 932, 934from a direct current path with the underlying voltage reference lines,while still providing the membranes 932, 934 with a relatively goodsignal ground. In the illustrated embodiment, the dielectric layer 936is shown disposed between each of the underlying voltage referencelines.

The membranes 932, 934 are coupled to a respective voltage source, whichcan include, for example, a DC bias, a ground reference, or a controlledor switched signal. For example, a voltage source can be coupled to acorresponding membrane using a variety of interconnection techniques,such as routing via a pad, an air bridge, and the like. For example,selected portions 938, 940 of the membranes 932, 934 can be formed atthe same time as forming of the membranes 932, 934. In one embodiment, aMEMS capacitor combining DC control and varying post spacing for themechanical conductor membrane can also be used.

FIG. 10A illustrates an example of an expected return loss for an RFattenuator using a MEMS capacitor. For example, as described earlier inconnection with FIG. 9, an RF signal can be configured to flow acrossthe MEMS capacitor. A horizontal axis indicates frequency withincreasing frequency to the right. A vertical axis indicates returnloss. The return loss corresponds to a ratio of an amplitude of thereflected wave to an amplitude of an incident wave and in FIG. 10A, theratio is further represented in decibels. As illustrated in FIG. 10A,trace 1002 corresponds to the expected return loss of the RF attenuatorwith the attenuator in an “off” position, that is, when the mechanicalconductor membrane 702 is in the low capacitance position illustratedfor example in FIG. 7D. As illustrated by the trace 1002, the expectedreturn loss is relatively low when the attenuator is “off,” so that theRF signal passes through the RF attenuator with the MEMS capacitor withrelatively low attenuation.

Other traces 1004, 1006, 1008 correspond to the return loss of the RFattenuator with the MEMS capacitor wherein the mechanical conductormembrane 702 is “pulled in” to a relatively high capacitance position asillustrated for example in FIG. 7E. The other traces 1004, 1006, 1008vary with respect to an amount of capacitance used in the estimation. Itwill be understood that the amounts of capacitance can vary depending onthe geometry of a capacitor and/or for a capacitor having multipleportions or multiple separate membranes that can be at least partiallyindependently actuated, for the amount of capacitance selected. Forexample, the capacitance corresponding to trace 1004 is greater thanthat used for trace 1006, which in turn is greater than the capacitanceused for the trace 1008. As illustrated in the example, the return lossof the attenuator at relatively low frequencies can vary with the amountof capacitance exhibited by the attenuator.

FIG. 10B illustrates an example of an expected insertion loss for an RFattenuator using a MEMS capacitor. The insertion loss corresponds to thereciprocal of the ratio of the signal power provided at an outputterminal of an RF attenuator to the signal power provided as an input toan input terminal of an RF attenuator. For example, the input and theoutput terminals can be on opposing ends of a signal conductor asdescribed earlier in connection with FIG. 9. A horizontal axis indicatesfrequency, with increasing frequency to the right. A vertical axisindicates insertion loss in decibels.

A trace 1012 corresponds to an expected insertion loss for an RFattenuator with a MEMS capacitor with the mechanical conductor membrane702 in a relatively low capacitance position illustrated, for example,in FIG. 7D. Other traces 1014, 1016, 1018 correspond to expectedinsertion losses for the RF attenuator when the mechanical conductormembrane 702 is in a relatively high capacitance position illustrated,for example, in FIG. 7E. The various traces 1014, 1016, 1018 correspondto expected insertion losses for varying amounts of capacitance. Thecorresponding capacitances for the trace 1014 is greater than thecorresponding capacitance for the trace 1016, which in turn is greaterthan the corresponding capacitance for the trace 1018. Also, asillustrated by the example of FIG. 3B, as the capacitance of the RFattenuator is changed, the resonant frequency f₀ of the RF attenuatorshould also change, and the insertion loss will typically be affected.This permits the insertion loss of an RF attenuator with a MEMScapacitor to be selected according to an amount of capacitance actuated.

For example, the resonant frequency f₀ of the RF attenuator is based atleast in part on the capacitance of the MEMS capacitor. The RFattenuator can be modeled by an RLC circuit 1102 as illustrated in FIG.11. For example, a first terminal 1104 can correspond to an inputterminal for the RF attenuator. A second terminal 1106 can correspond toan output terminal. The first terminal 1104 and the second terminal 1106can be at opposing ends of the signal conductor. Resistances R model theresistance of the signal conductor. The RLC circuit 1102 models theselectable capacitance to signal ground provided by the MEMS capacitor.

Variation in the capacitance of the RF attenuator correspondingly variesthe resonant frequency f₀ of the RF attenuator. Accordingly, theresonant frequency of the variable attenuator can be controlledaccording to the control voltages for the MEMS capacitor applied to theRF attenuator. This permits, for example, an RF attenuator with a MEMScapacitor to be implemented as a tunable filter, wherein the resonantfrequency of the filter can be modified or selected by a control circuitwhich controls one or more voltage levels applied to actuate one or moreportions or membranes of the MEMS capacitor. In addition, one or more RFattenuators exhibiting different resonant frequencies can be implementedas a band pass or a notch filter.

FIGS. 12A, 12B, and 12C illustrate examples of simplified equivalentcircuits for a MEMS capacitor. The membrane of the MEMS capacitorC_(MEMS) 1202 can be coupled to ground as illustrated in FIG. 12A. Acontrol bias selectively controls the amount of capacitance of the MEMScapacitor C_(MEMS) 1202 by selectively pulling in the membrane. One ormore signals can be capacitively coupled via a coupling capacitor CC1204 to the MEMS capacitor C_(MEMS) 1202. It will be understood that theinput signal and the output signal can be separately coupled to the MEMScapacitor C_(MEMS) 1202.

FIG. 12B illustrates where at least one membrane of a MEMS capacitor isnot directly coupled to a DC ground. This permits independent control ofthe membranes of a MEMS capacitor having a plurality of membranes. Forexample, the configuration described earlier in connection with FIG. 8Acan be used to place a control bias on a membrane. A first membrane hasa selectable capacitance C_(MEMS1) 1212 which is at least partiallycontrolled by a control bias on the signal conductor (control A) and acontrol bias on the membrane (control B). A capacitance C_(S) 1216 canbe used to provide a signal ground for the first membrane. Such that thecapacitance C_(S) 1216 should not significantly affect the seriescombination of capacitance to signal ground, it will be understood thatthe amount of the capacitance C_(S) 1216 should be relatively highcompared to the amount of capacitance selectable from the selectablecapacitance C_(MEMS1) 1212.

A second membrane has a selectable capacitance C_(MEMS2) 1214. In theillustrated circuit, the second membrane is coupled to ground andactuation is controlled by the control bias on the signal conductor(control A). One or more coupling capacitors C_(C) 1218 can again beused to isolate the control bias from the signals. In one embodiment,the signal flows through a signal conductor that is common to differentmembranes modeled by selectable capacitance C_(MEMS1) 1212 andselectable capacitance C_(MEMS2) 1214. The second membrane can also beindependently biased (control C) and AC coupled to a signal ground via acoupling capacitor C_(S) 1218 as illustrated in FIG. 12C. In addition,there can be additional membranes with independent control biases.

FIGS. 13A to 131 illustrate a process to fabricate a MEMS capacitor,such as the MEMS capacitor illustrated in FIGS. 7D and 7E. It will beappreciated by the skilled practitioner that the illustrated process canbe modified in a variety of ways. Advantageously, semiconductorfabrication techniques can be used to fabricate the MEMS capacitor. Forexample, in another embodiment, various portions of the illustratedprocess can be combined, can be rearranged in an alternate sequence, canbe removed, and the like.

FIGS. 13A to 13I illustrates cross sections of a MEMS capacitor invarious stages of fabrication. FIG. 13A illustrates a substrate assembly1300 having conductive lines for the signal conductor 1302 and forvoltage reference lines 1304, 1306 formed thereon. For example, theconductive lines can be formed by blanket deposition of a conductivematerial, such as aluminum, and by photoresist patterning and etching.In addition, where independent actuation of membranes is desired byseparate control biases, at least one of the voltage reference lines1304, 1306 can further be patterned into separate conductive lines.

FIG. 13B illustrates forming an insulating layer 1308 on the substrateassembly 1300. The insulating layer 1308 can be formed from a variety ofmaterials, such as silicon oxide, silicon nitride, aluminum oxide andthe like. Photolithography techniques can be used to pattern theinsulating layer 1308 to leave portions 1310 of the insulating layerbehind where desired as shown in FIG. 13C. In FIG. 13C, the insulatinglayer is shown left on the signal conductor 1302. Where independentmembrane actuation is desired, the insulating layer can also be left onat least some of the voltage reference lines.

A blanket deposition of a sacrificial material 1312 is illustrated inFIG. 13D. This sacrificial material 1312 is eventually removed. Examplesof sacrificial materials that are appropriate to use include silicon andmolybdenum. Other materials will be readily determined by one ofordinary skill in the art. The sacrificial material 1312 is patternedfor posts 1314 and for anchoring points 1316 for the membrane as shownin FIG. 13E.

FIG. 13F illustrates a blanket deposition of a material 1318 for posts.For example, the posts can be made from a photosensitive polymermaterial, that is, photoresist. For example, the photosensitive polymermaterial can be patterned to form the posts by light exposure through aphoto mask and chemical development. Accordingly, the post material 1318is removed and/or reduced in thickness from selected areas. For example,FIG. 13G illustrates removal of the post material from the anchor points1316 for the membrane. Optionally, a chemical mechanical polishing canbe performed to provide a flatness to an upper surface of the posts 1320and the sacrificial material (not shown).

FIG. 13H illustrates blanket depositing of a material 1322 to form themechanical conductive membrane. For example, aluminum can be depositedon the substrate assembly. The material 1322 can be patterned to formseparate membranes and the like. In addition, relatively small holes canbe patterned in the material 1322. These holes permit a gas etchant toaccess and remove remaining portions of the sacrificial material 1312from underneath the membranes, resulting in the structure illustrated inFIG. 13I.

Various embodiments have been described above. Although described withreference to these specific embodiments, the descriptions are intendedto be illustrative and are not intended to be limiting. Variousmodifications and applications may occur to those skilled in the artwithout departing from the true spirit and scope of the invention asdefined in the appended claims.

1. An apparatus having a selectable amount of capacitance, the apparatuscomprising: at least two electrodes at least one of which iscontrollably movable with respect to the other to provide adjustabilityof a gap defined between the at least two electrodes, wherein at leastone of the electrodes carries an RF signal, wherein the at least twoelectrodes comprise a first portion and a second portion; wherein thefirst portion of at least one of the electrodes is configured to betensioned by a first plurality of posts with a first post spacing,wherein the first plurality of posts are located between a substrate andat least one of the electrodes; and wherein the second portion of atleast one of the electrodes is configured to be tensioned by a secondplurality of posts with a second post spacing, wherein the secondplurality of posts are located between the substrate and at least one ofthe electrodes.
 2. The apparatus as defined in claim 1, furthercomprising a circuit configured to selectively apply a DC bias betweentwo electrodes, at least one of which is the movable electrode, suchthat the circuit adjusts a size of the gap between the two electrodes.3. The apparatus as defined in claim 1, further comprising: a display; aprocessor that is in electrical communication with said display, saidprocessor being configured to process image data; and a memory device inelectrical communication with said processor.
 4. The apparatus of claim3, further comprising a driver circuit configured to send at least onesignal to said display.
 5. The apparatus of claim 4, further comprisinga controller configured to send at least a portion of said image data tosaid driver circuit.
 6. The apparatus of claim 3, further comprising animage source module configured to send said image data to saidprocessor.
 7. The apparatus of claim 6, wherein said image source modulecomprises at least one of a receiver, transceiver, and transmitter. 8.The apparatus of claim 3, further comprising an input device configuredto receive input data and to communicate said input data to saidprocessor.
 9. A capacitor having a selectable amount of capacitance, thecapacitor comprising: means for carrying an RF signal, the carryingmeans having a controllably adjustable gap; and means for tensioning atleast part of the carrying means, wherein the carrying means comprises afirst portion and a second portion, wherein the first portion isconfigured to be tensioned by a first plurality of posts with a firstpost spacing, and wherein the second portion is configured to betensioned by a second plurality of posts with a second post spacingwherein the carrying means comprises at least two electrodes, at leastone of which is movable with respect to the other.
 10. The capacitor asdefined in claim 9, wherein the tensioning means comprises one or moreposts disposed adjacent to a movable portion of the carrying means. 11.The capacitor as defined in claim 9, wherein the carrying meanscomprises a deformable membrane.
 12. The capacitor as defined in claim11, wherein the carrying means additionally comprises a circuitconfigured to selectively apply a DC bias.
 13. The capacitor as definedin claim 12, wherein the carrying means comprises a plurality of gapsbetween a conductive line configured to carry an RF signal and aconductive membrane.
 14. The capacitor as defined in claim 12, whereinthe carrying means further comprises: means for selecting a DC bias; andmeans for applying the DC bias to the at least two electrodes.
 15. Thecapacitor as defined in claim 14, wherein the selecting means comprisesa processor or a logic circuit.
 16. The capacitor as defined in claim15, wherein the applying means comprises a switch.
 17. A method ofselecting capacitance, the method comprising: adjusting a gap between atleast two electrodes, wherein at least one of the electrodes carries anRF signal, wherein the at least two electrodes comprise a first portionand a second portion; and wherein the first portion is configured to betensioned by a first plurality of posts with a first post spacing, andwherein the second portion is configured to be tensioned by a secondplurality of posts with a second post spacing wherein adjusting the gapcomprises adjusting a plurality of gaps between a conductive lineconfigured to carry the RF signal and a plurality of electrodes.
 18. Themethod as defined in claim 17, wherein adjusting the gap furthercomprises: selecting a DC bias; and applying the DC bias to the at leasttwo electrodes.
 19. A method of manufacturing a capacitor having aselectable capacitance, the method comprising: forming a firstelectrode; forming a second electrode such that it is movable withrespect to the first electrode to provide adjustability of a gap definedbetween the first electrode and the second electrode, wherein the firstand second electrodes comprise a first portion and a second portion; andwherein the first portion is configured to be tensioned by a firstplurality of posts with a first post spacing, and wherein the secondportion is configured to be tensioned by a second plurality of postswith a second post spacing.
 20. The method as defined in claim 19,further comprising forming the first electrode from a non-transparentmaterial.
 21. A capacitor produced in accordance with the method definedin claim 19.